<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>
<body>
  
  <button>按钮</button>
  <script>
    // 内置函数(第三方库): 根据一些经验
    // 1.定时器
    setTimeout(function() {
      console.log("定时器函数:", this)  //this=>window
    }, 1000)

    // 2.按钮的点击监听
    var btnEl = document.querySelector("button")
    btnEl.onclick = function() {
      console.log("btn的点击:", this)  //this=>DOM对象
    }

    btnEl.addEventListener("click", function() {
      console.log("btn的点击:", this)
    })

    // // 3.forEach
    var names = ["abc", "cba", "nba"]
    names.forEach(function(item) {
      console.log("forEach:", this)
    })  //this=>window
    names.forEach(function(item) {
      console.log("forEach:", this)
    }, "aaaa") //this=>String('aaaa') 显式绑定this


  </script>

</body>
</html>